This invention relates to the analysis of trace metal impurities in semiconductor materials. More specifically this invention relates to an improved technique for determining the content of these trace metals in the low to sub-part per billion, atomic, (ppba) range.
Because of the extremely high performance demands of the electronics industry, semiconductor materials of extremely high purity are required. Analytrical techniques to characterize extremely low levels (ppba) of trace metals are becoming a necessity. The development of appropriate analytical techniques has been ongoing for many years. As an example, trace metal analysis in semiconductor silicon at low (ppba) levels has been attempted with many analytical techniques. Examples of such techniques are mass spectrometry, neutron activation analysis, and atomic absorption spectrometry.
Spark source mass spectrometry, as noted in Associated Electrical Industries Ltd. Publication 2030/A16, Oct.,1960, has been utilized to analyze such metals as chromium, copper, iron, and nickel to levels down to about 100, 50, 300, and 500 ppba, respectively. This technique only measures point samples and not bulk samples. This technique involves a very small area of a sample surface which may not be representative of the bulk of the sample.
One of the more sensitive analytical techniques is neutron activation analysis. This technique is described in several references, including: Martin, Semiconductor Silicon, Ed. by R. R. Haberecht, p. 547 (1969); Heinen et al., Anal. Chem., 38 (13), p. 1853 (1966); and Thompson et al., Anal. Chem., 30(6), p. 1023 (1958). While this technique is quite sensitive, a large neutron-generating radiation source is necessary. Additionally, several weeks can be required to complete the monitoring of the radioactive decay of the nucleides generated. Thus, this technique is both expensive and time-consuming.
Atomic absorption spectrometry for the analysis of trace metals in silicon has been improved by going from a flame technique tp flameless graphite furnace technique. Sensitivity in the picogram range is now possible. This flameless graphite furnace technique has been used by many investigators in the following references: Stewart et al., Analyst, 108, p. 1450 (1983); Taddia, Anal. Chim. Acta, 142, p. 333 (1982); and Fuller, Anal. Chim. Acta, 62, p. 261 (1972). This technique requires that a sample be placed in solution before analysis. More significantly, because of the low levels of metal for which detection is being attempted, very meticulous, time-consuming procedures must be applied to prevent background contamination from masking the analysis being attempted.
In all of the above techniques contamination in sample preparation can mask results when semiconductor samples of low trace metal content are analyzed.